A first guess would be that these diets are passing over aggregate compounds and compatible metabolic waste: misfolded proteins, lipfuscin, oxidized lipids, and

A first guess would be that these diets are passing over aggregate compounds and compatible metabolic waste: misfolded proteins, lipfuscin, oxidized lipids, and so forth, and enough of those escape the stomach to be incorporated into tissues to do harm. That said, 10% for mice is well within the "try it again in a different lab, get a different result" margin of error for this sort of study.